We will utilize already-existing data that have been obtained by the NHLBI- sponsored Resuscitation Outcomes Consortium (ROC). Although the ROC generates vast quantities of resuscitation data, ROC investigators have been unable to maximize these due to the labor-intensive nature of the type of work we propose. Studying patients who experience out-of-hospital cardiac arrest, we will determine the incidence of re-arrest in patients who achieve a return of spontaneous circulation (ROSC) during the prehospital phase of their care. We believe that by developing a better understanding of the incidence and outcomes of re-arrest, the electrocardiographic (ECG) characteristics that indicate impending re-arrest, and clinical variables that affect re-arrest, more patients can be helped to survive.